The present invention relates to an X-ray mask blank used for X-ray lithography and a method of manufacturing the X-ray mask blank.
In the semiconductor industries, a photolithography process is used for forming a minute pattern of integrated circuit on a silicon wafer or the like. In this event, the minute pattern is transcribed on the silicon wafer or the like by the use of a visible light and/or an ultraviolet ray as an exposing electromagnetic wave.
In accordance with a progress of a large integration technique in semiconductor devices, such as an ultra large scale integrated circuit, a high integration technique is required to transcribe more minute pattern to the silicon wafer or the like than that made by the use of the visible light and/or ultraviolet light.
In reply to a demand of a technique of transcribing a more minute pattern by the use of the visible light and/or the ultraviolet ray, an X-ray lithography process is progressed.
The X-ray lithography process uses an X-ray of a wavelength shorter than that of the visible light and/or the ultraviolet ray and is tried for transcribing such a very minute pattern to the silicon wafer or the like.
In the X-ray lithography process, use is made of an X-ray mask comprising the very minute pattern of an X-ray absorber material supported by a silicon substrate through an X-ray transparent film on the silicon substrate. The X-ray transparent film is generally made of a silicon carbide while the X-ray absorber material is formed by an amorphous material containing tantalum (Ta). With this X-ray mask, exposure is carried out through the X-ray mask onto the silicon wafer or the like to transcribe the the very minute pattern of the X-ray absorber material to the silicon wafer or the like. Such a film of the X-ray absorber material will be called an X-ray absorber film.
Such an X-ray mask is obtained by patterning an X-ray mask blank which comprises the X-ray absorber film of tantalum (Ta), tungsten (W), or a compound containing metals thereof.
On manufacturing the X-ray mask blank, the X-ray absorber film is deposited on the X-ray transparent film by sputtering. Such an X-ray mask blank has been proposed in Japanese Unexamined Patent Publication No. Hei 2-192116, namely, 192116/1990 and has an X-ray absorber film of Ta and B.
In the mean while, a high accuracy of position has been required. Such a high accuracy is accomplished by reducing a positional distortion on the X-ray mask. A high internal stress of the X-ray absorber film induces the positional distortion. The positional distortion is strongly influenced by an internal stress of a mask material.
In this event, the X-ray absorber film is required to have a small internal stress in order to minimize the positional distortion. For example, a distortion should fall within 22 .mu.m for the mask used for a one giga bits dynamic random access memory (1G-DRAM) formed by a design rule of 0.18 .mu.m.
As a technique to minimize the internal stress of the X-ray absorber film, a method is disclosed in JPA-150324/1989. The method of JPA-150324/1989 comprises steps of forming an X-ray absorber material of Ta having a tensile stress of about 1.times.10.sup.9 dyn/cm.sup.2 (100 MPa) on an X-ray transparent film formed on a substrate by the use of a sputtering apparatus and, thereafter, heating the X-ray absorber material for a predetermined time period to form an X-ray mask capable of lowering to about 1.times.10.sup.8 dyn/cm (10 MPa).
Discussion has been often made to measure the internal stress accurately. In a conventional measurement method of the internal stress, only a mean stress is calculated as the internal stress in a central position, specifically, in which a radius of curvature of the substrate is measured before and after forming the film. Therefore, the mean stress is measured as the internal stress only in the central position of the silicon substrate.
Accordingly, JPA-150324/1989 never considers a uniformity of stress over the whole of the pattern area of the X-ray mask. In the X-ray mask proposed in JPA-150324/1989, Ta film structure practically has a tensile stress of about 100 MPa after formed and, apparently, has a prismatic crystal structure.
When the X-ray absorber film has a prismatic crystal structure, it has been found out according to the inventor's observation that an edge shape of pattern becomes rough and worse in forming the minute pattern. In order to finely delineate the X-ray absorber film with a minute pattern not more than 0.18 .mu.m, it has been also found out that the X-ray absorber film should be in a fine crystalline state or amorphous structure.
Therefore, the Ta film of JPA-150324/1989 can not be applied to form a pattern of IG-DRAM.
In order to form the Ta film of a fine crystal structure, a film stress after forming should be put in a compressive state. When the Ta film is put into the compressive stress, the film is not kept in a low stress during annealing.
The followings are confirmed by a verification experiment of the present inventor. A nonuniform stress distribution causes a pattern distortion to occur on the X-ray mask even if the mean stress of the X-ray absorber film falls within a range not more than 10 MPa. Therefore, a required accuracy of position is not achieved on the X-ray absorber film because of a nonuniform stress of the pattern area.
Preferably, the X-ray absorber has a required accuracy of position over a wide pattern area of at least 25 mm square.
In the meanwhile, it is recent tend that the stress can be accurately measured by a measuring apparatus.
For example, a stress measurement system developed by NTT-AT Inc. can accurately measure a stress distribution by measuring a radius of curvature in a substrate.
The other method in which the stress distribution can be accurately measured is called Bulge method. In the Bulge method, a differential pressure is imposed on the membrane to an amount of deformation. By both of the method above-mentioned, the stress distribution can be measured more accurately inside the substrate.
When various absorber films are evaluated by stress distributions, each of the absorber films formed by methods disclosed in JP-A-192116/1990 and JP-A-150324/1989, has a nonuniform stress distribution and, therefore, can not satisfy a required accuracy of position.
Therefore, it is understood that the nonuniform distribution of the film thickness brings about the positional distortion in a nonuniformity of the stress.